SQL Bug in "Minus" in correlated subquery presence of index (11.2.0.1.0)

Question

SQL Bug in "Minus" in correlated subquery presence of index
(Oracle Database 11g Release 11.2.0.1.0)
Below, there is a small example that shows the bug. Further below,
there are some more comments.
drop table Country;
create table Country
(code VARCHAR2(4) constraint countrykey PRIMARY KEY,
name VARCHAR2(35));
-- if the key constraint is not given, the bug does not occur
drop table City;
create table City
(name VARCHAR2(35),
country VARCHAR2(4),
population number);
drop table Locatedon;
create table Locatedon
(city VARCHAR2(35),
country VARCHAR2(4),
island VARCHAR2(35));
insert into country values('E','Spain');
insert into country values('F','France');
insert into country values('S','Sweden');
insert into country values('GB','Sweden');
insert into city values('Ajaccio','F',53500);
insert into city values('Paris','F',2152423);
insert into city values('Palma','E',322008);
insert into city values('Madrid','E',3041101);
insert into city values('Stockholm','S',711119);
insert into city values('London','GB',6967500);
insert into locatedon values('Ajaccio','F','Corse');
insert into locatedon values('Palma','E','Mallorca');
insert into locatedon values('London','GB','Great Britain');
-- all countries that have a city that is not located on
-- some island: should be E, F, S.
Select c.name
From country c
Where exists
((Select name
From city
Where city.country=c.code)
minus
(Select city
From locatedon
Where locatedon.country=c.code)
-- wrong answer: only Sweden; Spain and France not in the answer!
select distinct country from
((Select name, country
From city)
minus
(Select city, country
From locatedon)
-- correct answer: E, F, S
Comments:
The bug has been found by students in our SQL course.
Using a larger database from that course, the bug can be reproduced
(same queries as above) at
http://www.semwebtech.org/sqlfrontend/
(wrong: 142 answers, correct: 154 answers)
During reducing it to a simple sample, there were some interesting
observations: trying with smaller and simpler tables (without the keys)
and synthetic data, the bug did not occur immediately. When
restating the query after about one day, the bug occurred. Obviously,
Oracle creates some index on its own in course of its internal
optimization that (or more exactly, its usage) exhibits the bug. The
query plan (showed in SQL Developer) was the same before and after.
Wolfgang

Answer 1

There's a typo in the test data - GB should presumably not be in Sweden. However....
the bug did not occur immediatelyIt's possible. But what would have almost certainly happened is that the execution plan DID change at some point. There are various reasons why it might not be immediate.
Obviously, Oracle creates some index on its own in course of its internal optimizationFar from obvious, what are you on about?
The query plan was the same before and afterBet you it wasn't.
A clear illustration of the issue and indication that it must be a bug is below.
Simply by hinting a different access method, we can change the result. Therefore, bug.
See [url http://support.oracle.com]Oracle Support and search for "wrong results".
Please raise with Oracle Support to get confirmation of bug.
There have been so many wrong results bugs recently, it's getting ridiculous.
It's a real issue, IMHO.
If you can't trust the DB to get your data right....
Note that the query plan is very much NOT the same and it is the difference in query plan which s that is the root cause of the bug.
SQL> select * from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
SQL> SELECT c.name
  2  FROM   country1 c
  3  WHERE  exists ((SELECT name
  4                  FROM   city1
  5                  WHERE  city1.country=c.code)
  6                  MINUS
  7                 (SELECT city
  8                  FROM   locatedon1
  9                  WHERE  locatedon1.country=c.code));
NAME
Sweden
SQL> SELECT /*+ full(c) */
  2         c.name
  3  FROM   country1 c
  4  WHERE  exists ((SELECT name
  5                  FROM   city1
  6                  WHERE  city1.country=c.code)
  7                  MINUS
  8                 (SELECT city
  9                  FROM   locatedon1
10                  WHERE  locatedon1.country=c.code));
NAME
Spain
France
Sweden
SQL> explain plan for
  2  SELECT c.name
  3  FROM   country1 c
  4  WHERE  exists ((SELECT name
  5                  FROM   city1
  6                  WHERE  city1.country=c.code)
  7                  MINUS
  8                 (SELECT city
  9                  FROM   locatedon1
10                  WHERE  locatedon1.country=c.code));
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 156929629
| Id  | Operation                    | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |            |     1 |    27 |    12  (25)| 00:00:01 |
|   1 |  NESTED LOOPS                |            |       |       |            |          |
|   2 |   NESTED LOOPS               |            |     1 |    27 |    12  (25)| 00:00:01 |
|   3 |    VIEW                      | VW_SQ_1    |     6 |    24 |    10  (20)| 00:00:01 |
|   4 |     MINUS                    |            |       |       |            |          |
|   5 |      SORT UNIQUE             |            |     6 |   138 |            |          |
|   6 |       TABLE ACCESS FULL      | CITY1      |     6 |   138 |     4   (0)| 00:00:01 |
|   7 |      SORT UNIQUE             |            |     3 |    69 |            |          |
|   8 |       TABLE ACCESS FULL      | LOCATEDON1 |     3 |    69 |     4   (0)| 00:00:01 |
|*  9 |    INDEX UNIQUE SCAN         | COUNTRYKEY |     1 |       |     0   (0)| 00:00:01 |
|  10 |   TABLE ACCESS BY INDEX ROWID| COUNTRY1   |     1 |    23 |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   9 - access("VW_COL_1"="C"."CODE")
Note
   - dynamic sampling used for this statement (level=4)
26 rows selected.
SQL> explain plan for
  2  SELECT /*+ full(c) */
  3         c.name
  4  FROM   country1 c
  5  WHERE  exists ((SELECT name
  6                  FROM   city1
  7                  WHERE  city1.country=c.code)
  8                  MINUS
  9                 (SELECT city
10                  FROM   locatedon1
11                  WHERE  locatedon1.country=c.code));
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 1378726376
| Id  | Operation            | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT     |            |     1 |    23 |    14  (15)| 00:00:01 |
|*  1 |  FILTER              |            |       |       |            |          |
|   2 |   TABLE ACCESS FULL  | COUNTRY1   |     4 |    92 |     4   (0)| 00:00:01 |
|   3 |   MINUS              |            |       |       |            |          |
|   4 |    SORT UNIQUE       |            |     1 |    23 |     5  (20)| 00:00:01 |
|*  5 |     TABLE ACCESS FULL| CITY1      |     1 |    23 |     4   (0)| 00:00:01 |
|   6 |    SORT UNIQUE       |            |     1 |    23 |     5  (20)| 00:00:01 |
|*  7 |     TABLE ACCESS FULL| LOCATEDON1 |     1 |    23 |     4   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter( EXISTS ( (SELECT "NAME" FROM "CITY1" "CITY1" WHERE
              "CITY1"."COUNTRY"=:B1)MINUS (SELECT "CITY" FROM "LOCATEDON1" "LOCATEDON1"
              WHERE "LOCATEDON1"."COUNTRY"=:B2)))
   5 - filter("CITY1"."COUNTRY"=:B1)
   7 - filter("LOCATEDON1"."COUNTRY"=:B1)
Note
   - dynamic sampling used for this statement (level=4)
27 rows selected.Just to show that it's related to query transformation:
SQL> SELECT /*+ 
  2             no_query_transformation
  3         */
  4         c.name
  5  FROM   country1 c
  6  WHERE  exists ((SELECT name
  7                  FROM   city1
  8                  WHERE  city1.country=c.code)
  9                  MINUS
10                 (SELECT city
11                  FROM   locatedon1
12                  WHERE  locatedon1.country=c.code));
NAME
Spain
France
Sweden
SQL> Edited by: Dom Brooks on Jun 30, 2011 2:50 PM